This invention relates to hydrofoil apparatus and, more specifically, this invention relates to hydrofoil apparatus for inclusion in any towed arrangement which, in order to fulfill its function, requires hydrodynamic lift as a component of the force that opposes the towing effort.
Known hydrofoil apparatus for towing fall generally into two use categories, but may also fall into both at once. The first category of use includes a wide variety of activities that require an object, or different types of equipment, to be towed through the water by a vessel or other towing point for purposes of, for example, performing special measurements, catching or positioning something. It is often important that the object or equipment being towed should not follow directly behind the point of tow but be pulled out by a diverter to one side or another, pulled downwards by a depressor, or even pulled upwards by an elevator, if the towing point is beneath the water surface. Examples of hydrofoil apparatus that can perform some of these roles have been variously referred to as paravanes; vanes; mono-wings; diverters; doors; otter boards or just otters; deflectors; depressors; elevators and kites.
The second category of use includes all those arrangements in which the effort generated by the hydrofoil apparatus is used to effect the towing point or vessel in some desirable way. These might, for example, include the role of a sea-anchor, when used to give some direction to a vessel""s drift; the role of a stabilizer used to stabilize a vessel in roll; the role of providing lateral resistance in a sailing arrangement such as that of the more conventional waterborne vessel that supports a rig of sails, or a more unusual airborne arrangement of aerofoil such as, for example; an autogyro; a hang-glider, kite or other winged craft; a paraglider; or a displacement vessel such as an airship or balloon.
A water-air interface is an extremely complex and difficult environment in which to operate towed hydrofoil apparatus. Typically, on reaching or breaking the water surface, most hydrofoil apparatus for towing will become unstable and cease to function as desired.
For those bodies and types of equipment that are required to be towed at or close to the water surface, a common practice has been to ensure that the hydrofoil apparatus remains fully immersed and at the desired running depth by means of a float on the surface. However, this does little to stabilize the hydrofoil apparatus in yaw and can itself be disruptive in rough water, so additional means are usually employed. Where towed equipment produces enough drag force, this can be used to stabilize the hydrofoil apparatus in yaw, but it is not usually desirable to introduce a constant drag force unnecessarily. Also, the capacity of a float to exercise control remains fixed, while the disruptive dynamic forces over which it is required to prevail grow in proportion to the square of the water speed. The size of float required, as speeds increase, would therefore grow out of all proportion producing excessive drag and becoming potentially unmanageable and dangerous to handle. Furthermore, the use of a float does not avoid the inevitability that the hydrofoil surfaces will become partially unwetted during their launch and recovery; a condition that is usually unstable and can produce difficult if not dangerous handling.
Many bodies and other types of equipment are not required to be towed at or close to the water surfaces when fully deployed, so would be hampered by a float. Nevertheless, they too must usually pass through the surface conditions during their launch and recovery, and in many cases it is also desirable that this can be carried out at some speed and through rough water.
For sailing it is desirable that a hydrofoil apparatus is towed at the water surface and often at considerable speed through rough water. It is also desirable that the same apparatus may be operated on either tack, which can be difficult to arrange if a float is employed for surface sensing.
To assist or take over the functions of a float completely, known hydrofoil apparatus have therefore been constructed with anhedral, to sense the water surface in a simple dynamic way. These known apparatus have a lower portion of hydrofoil surface which is orientated to give a depressing component of lift and an upper portion orientated to give an elevating component of lift. These two lift components therefore act in parallel and opposite directions away from each other. The apparatus can then be adjusted in roll through bridle adjustments, until the elevating and depressing lift components are in balanced opposition, while a part of its elevating portion pierces the water surface to remain in reserve. Should there then be a gain or loss in wetted surface area, the resultant of the lift components provides a restoring force that works to restore the apparatus to the desired running depth.
Unfortunately though, the opposing lift components also tend to form couples that seek to turn the apparatus in the direction in which it is moving at any one moment, during its surface sensing depth corrections. If, therefore, the apparatus is responding to either an elevating or depressing lift resultant, it tends to turn upward, or downwards, respectively, towards the water surface. Further to this, the elevating and depressing portions experience changes in their angles of incidence which are accompanied by a variety of possible alterations in their lift to drag ratios. These can have the effect of redistributing its lift and drag such that the lift and towing force resultant, and the drag vector are separated across the direction of movement at any one moment. This gives rise to further couples that either work with or against the opposed lift component couples, depending on the angles of incidence at which the hydrofoil surfaces had been working. A common result is that of repeated and alternating turns towards the surface, sometimes developing into a marked or even violent xe2x80x9cporpoisingxe2x80x9d action.
Furthermore, if the apparatus should suffer disorientation due, for example, to an acquired drag force from weed, debris or from grounding, the surface sensing capability can be overcome, due to large changes, of an opposite nature, between the angles of incidence of the elevating and depressing portions. This can cause the apparatus to turn and jump from the water, dive beneath the vessel or, in the case of grounding, dive precipitously into the bottom.
The addition of a short stabilizing tail works with the opposed lift component couples to support any turns towards the water surface, which is unhelpfull. However, as the tail is lengthened, this support becomes increasingly less, tending more to support the maintainance of a fixed orientation, with respect to the general direction of advance of the hydrofoil apparatus, so obliging it to execute its surface sensing with a more side-slipping action. The longer the tail, therefore, the more turns towards the surface and xe2x80x9cporpoisingxe2x80x9d are suppressed. However, while this modification of behaviour is appropriate, it is found in practice to be insufficient, unless the tail is unacceptably long.
A further disadvantage can arise if a bridle member parts and, as a consequence, the apparatus adopts completely the wrong orientation. Due to the anhedral relationship of the hydrofoil surfaces, the apparatus can then behave much as a spinner does on a fishing line, causing considerable entanglement and further loss or damage.
For many uses it is important that the drag of a towed hydrofoil apparatus be reduced as much as possible, and particularly so for sailing. An immersed tow-line and bridle members can generate excessive drag if not satisfactorily faired. However, the effectiveness of known cable fairings that are used in a much wider context than just the present invention is limited because they are designed to feather freely about a towed cable. Consequently, a bridle member, tow-line or towed cable that is faired in this a must have a circular cross-section which imposes a lower limit to the thickness of fairing section that can be used around it. This in turn imposes a limit on the degree of drag reduction that is possible.
It is an aim of the present invention to provide hydrofoil apparatus which may be designed and or adjusted to operate at a wide range of speeds and angles of incidence, either deeply submerged or at the water surface while sensing the water surface dynamically, in smooth as well as rougher water, to one or to either hand of the towing point, and which decreases or eliminates the above mentioned disadvantages of simple anhedral surface sensing.
Accordingly, the present invention, in one non-limiting embodiment, provides hydrofoil apparatus comprising a first hydrofoil member having chord and span dimensions and positive hydrodynamic pitching moments, a second hydrofoil member having chord and span dimensions and positive hydrodynamic pitching moments, connection means for connecting the first and second hydrofoil members together such that they are able to articulate about the connection means, at least first and second bridle members which are for enabling the hydrofoil apparatus to be towed and are such that the first bridle member is articulately attached at one end to an outer end portion of the first hydrofoil member thereby forming a first pitching axis, the second bridle member is articulately attached at one end to an outer end portion of the second hydrofoil member thereby forming a second pitching axis, the first and second pitching axes forming an angle such that a component of hydrodynamic lift generated by the first hydrofoil member and a component of hydrodynamic lift generated by the second hydrofoil member act in parallel directions away from each other, and regulation means by which the angle formed by the first and second pitching axes is regulated.
The connection means may, in some embodiments of the present invention, comprise no more than one or a series of loose but captive links and/or flexible members of low torsional resistance, provided that the first and second hydrofoil members are permitted sufficient freedom to pitch.
The first and second hydrofoil members of the present invention have similar functions to the anhedral portions of a simple anhedral hydrofoil apparatus in that components of their hydrodynamic lift act in parallel and opposite directions away from each other. They differ, however, in that they have freedom to pitch about their pitching axes and have positive hydrodynamic pitching moments by which they each seek to adopt and maintain particular angles of incidence. When, therefore, the hydrofoil apparatus of the present invention acquires drag from attached weed, debris or from grounding, that would destabilize the simple anhedral hydrofoil apparatus, each hydrofoil member is able to adopt the appropriate angles of incidence that are required to maintain a balance in their opposing lift components. The hydrofoil apparatus therefore adopts a particular angle of sweep at which the couple formed by the horizontal separation of the opposing lift components is equal and opposite to that introduced by the drag force, enabling the hydrofoil apparatus to continue in the same general direction of advance, though with a changed orientation.
The angle formed by the first and second pitching axes requires at least some regulation by the regulation means because the most efficient hydrofoil apparatus will be that which has the greatest angle that is consistent with the minimum anhedral necessary for satisfactory surface sensing. Without regulation that sets a minimum angle, the angle adopted by the pitching axes would become considerably less than that desired and an uncertain variability would interfere with the normal functioning of the hydrofoil apparatus in several respects. However, it can be desirable that the minimum angle permitted is variable, and it can also be desirable that the regulation means permits a free increase of angle, somewhat above a minimum. If, then, a bridle member should part, causing the hydrofoil apparatus to adopt completely the wrong orientation, its anhedral is free to decrease or even pass beyond 180 degrees to a dihedral angle, so lessening or avoiding further damage and entanglement due to spinning.
The regulation means may include a third bridle member that is articulately attached at one end to the connecting means or to the inner end portions of the first and second hydrofoil members at locations that lie substantially on their pitching axes. Alternatively or as well the regulation means may include at least one strut, which may be hydrodynamically faired, having a first end which is articulately connected to the first hydrofoil member at a location that lies substantially on the first pitching axis and is displaced from the connecting means and a second end that is articulately connected to the second hydrofoil member at a location that lies substantially on the second pitching axis and is displaced from the connecting means. In order then to avoid spinning, a free increase of the regulated angle is permitted, for example, when; the distance between the first and second attached ends of the strut is free to increase above a certain minimum; and/or at least one attached strut end, is free to move in a generally spanwise direction, away from the outer end of its respective hydrofoil member, but is moved to an outer spanwise limit by the strut, when it comes under compression, the strut end/s then becoming substantially confined in a chordwise direction.
Alternatively or as well, the regulation means may include regulation that is provided in conjunction with the connection means. This occurs when the connection means is provided with a first connection axis about which the first hydrofoil member turns and a second connection axis about which the second hydrofoil member turns, the first and second connection axes being coaxial with the first and second pitching axes. In this case, in order to avoid spinning, a free increase of the regulated angle is permitted, for example, when the connection means includes at least one intermediate connecting member which turns about the first and/or second connecting axis, and which is articulately connected to its respective hydrofoil member such that a free increase of the regulated angle that lies to the pressure sides of the hydrofoil members is possible.
There is a further advantage if the regulation means will permit the first and second hydrofoil members to fold together, but only with their suction surfaces facing each other, the hydrofoil members having passed through the angles of anhedral as well as dihedral. This may be provided for in the same ways as described above for providing a free increase of the regulated angle, but with the range of freedom being appropriately extended. With this facility the normal operation of the hydrofoil apparatus remains unaltered, but it becomes possible to fold the hydrofoil members together, for ease of stowage and handling.
For many embodiments of the present invention it is a desirable control feature that the positive pitching moments of the hydrofoil members may be brought into opposition with one another. Such opposition means provides a reciprocal relationship by which an increase or decrease in the angle of incidence achieved by one hydrofoil member imposes a decrease or increase, respectively, on that which can be achieved by the other. The opposition means may be provided by a strut, as described above for the regulation means, except that its ends are attached to their respective hydrofoil members at locations that are displaced backwards from their respective pitching axes. The strut then still provides regulation as well, to the extent that it determines the minimum angle that the regulated angle may adopt.
Alternatively or as well, the connection member may provide opposition means in an equivalent way to that provided by a strut, as described above. For this alternative, the first and second connection axes are instead arranged to diverge backwards from the first and second pitching axes, respectively, as they reach towards the outer ends of their respective hydrofoil members, instead of being coaxial with them. The connection member then still provides regulation as well, to the extent that it determines the minimum angle that the regulated angle may adopt.
As with the simple anhedral hydrofoil apparatus, the hydrofoil apparatus of the present invention may be adjusted in role, to sense the water surface by adjusting the relative lengths of its bridle members. When the hydrofoil apparatus rises and falls, during surface sensing, the opposing lift components would, like the simple anhedral apparatus, tend to give rise to a xe2x80x9cporpoisingxe2x80x9d action.
However, the hydrofoil apparatus of the present invention does not normally employ a stabilizing tail. It is instead arranged that its lift and drag are redistributed such that, during normal operation, the resulting couples work against any opposed lift component couples to maintain its orientation with respect to its general direction of advance and not its direction of movement at any one moment. The hydrofoil apparatus therefore conducts its surface sensing movements with a side-slipping action. This may be achieved through the addition or removal of drag in appropriate ways. For example; at least one controllable drag rudder may be employed. Alternatively or as well, at least one of the hydrofoil members of a hydrofoil apparatus may have at least one end portion that includes at least one separate, full or part chord of hydrofoil surface which is orientated such that when the end portion is trailing, the hydrodynamic pitching moment of that portion is higher than when it is leading. The drag associated with generating a positive pitching moment is thereby increased when trailing. Furthermore, if each of the opposite end portions of the hydrofoil member have similar characters in this respect, drag is both removed from its leading end portion and added to its trailing end portion with little or no change to the pitching moment of the hydrofoil member as a whole. This is not necessarily desirable, however, when the hydrofoil apparatus is adjusted to have a small unwetted portion piercing the water surface. In this case, small changes of immersion would be accompanied by large changes of pitching moment. The separate or part chord hydrofoil surface of such a portion may therefore be arranged to have little or no influence over the hydrofoil member""s pitching moments, except when the outer end portion is trailing. To achieve this, it may be articulately mounted on its outer end portion and permitted to self feather, to its apparent water flow, when its end portion is leading, and only become active in generating positive pitching moments when its end portion is trailing.
Alternatively or as well, it can be arranged that, during surface sensing depth corrections, the lift to drag ratio of at least one of the hydrofoil members is altered by causing its angle of incidence to change appropriately, which will have the consequence of altering the distribution of lift as well as of drag, for the whole immersed apparatus.
When the distribution of lift and of drag for the whole hydrofoil apparatus is altered through a change in the angle of incidence of at least one of its hydrofoil members, it is not only the redistribution of lift and drag between the two hydrofoil members that determines the outcome, but also any redistribution that occurs between different portions of the individual hydrofoil member. The nature of this redistribution depends upon the lift to drag characteristics of the portions of hydrofoil member concerned. For example, an elevating hydrofoil member having considerable twist (in the form of wash-out) may increase its angle of incidence when, in the course of sensing the water surface, its degree of immersion is increased. While thereby maintaining a comparatively high lift to drag ratio on the portion that, at any one moment, is operating just below the water surface, its portion that is most deeply immersed experiences a marked increase in its angle of incidence, giving it high lift but also very high drag. This usefully redistributes both lift and drag as well as increasing the restoring force beyond that due to the increase of immersed area alone.
The distributions of both lift and drag will also change in characteristic ways with changes in the hydrofoil member""s angle of sweep with respect to its apparent water flow. Furthermore, if the hydrofoil member has movable control surfaces, or it deforms under load, its characteristic responses to the above mentioned experiences be altered.
The hydrofoil member acquires its lift to drag ratio characteristics from all aspects of its form. It may, for example, have straight or concave and convex surfaces along its span; be twisted in one or both hands; be of constant or varied chord; be of straight, curved or irregular planform; be of constant or varied cross-section along its span and be single-plane or multi-plane and may also have at least one separate control surface. Also, the first and second hydrofoil members need not necessarily be the same or mirror each other.
Similarly, the hydrofoil member acquires its pitching moment characteristics from all aspects of its form, as exampled above for its lift to drag characteristics. At least one control surface and/or deformation under load may be used to change the characteristic hydrodynamic pitching moments of at least one of the hydrofoil members. They may also be influenced by changes in immersion and/or angle of sweep.
The angles of incidence that the hydrofoil members adopt may be further influenced by controlling the strength of opposition, since, in addition to its reciprocal nature, the opposition means provides a differential mechanism by which dual control can be exercised over the angles of incidence that the first and second hydrofoil members are permitted to achieve. This occurs, for example, when the strength of opposition is controlled by varying the distance between the attached ends of a strut that is providing the opposition means; by varying the position of attachment of at least one of the attached strut ends on its respective hydrofoil member; by varying the angle at which at least one of the connection axes diverges from its respective pitching axis; and/or by varying the regulated angle.
Pitching limitation means, whereby at least one of the first and second hydrofoil members has only limited freedom to pitch, are desirable for many embodiments of the present invention. This is particularly so whilst the apparatus is being operated at very low speeds and angles of incidence, when the pitching and stabilizing hydrodynamic forces are low. Pitching limitation means may be provided, for example; by limiting a hydrofoil member""s freedom to pitch about its respective connection axis; or, if a strut is present, the shape of the strut attachment end, and of the hydrofoil member, over their respective surfaces that come to bear against each other, may be such that pitching is limited in the desired way. For example, a protruberance reaching forwards and or backwards from one or both of a strut""s attachment ends may be so shaped that it comes to bear on the respective hydrofoil member, inhibiting further decrease or increase in pitch beyond the desired limit/s. As a further example, the bearing surface of the hydrofoil member may be of a socket nature, to receive the strut end, its movement being restricted as desired within the socket.
It will be appreciated from the description above that there are many ways of imparting and configuring the many characteristics of the hydrofoil members and their interaction, in order to fulfill the wide range of requirements found within the different types of use to which the hydrofoil apparatus may be put. For example, the operating environment at a water-air interface is asymmetrical and can therefore, in some respects, be best met with an asymmetrical hydrofoil apparatus. In order, therefore, to operate efficiently at the surface and to either hand, such asymmetry may need to be reversable. However, none of these many ways would depart from the underling principles by which the hydrofoil apparatus functions.
Many embodiments of the present invention will include at least one means of controlling their character and behaviour, to suit different purposes and circumstances. The adjustments needed may be controlled by, for example, any combination of the following; by pre-setting; by remote control, by the control of surface and/or bottom sensing equipment; by the control of pressure sensing equipment; by the control of motion sensing equipment; and/or by the control of load sensing equipment.
One form of pre-setting and/or remote control may include bridle adjustments. As described above, the primary controlling effect of altering the relative lengths of bridle members is to alter the orientation of the first and second hydrofoil member""s lift vectors, and so vary the elevating and depressing lift components.
However, in altering the bridle member lengths the geometry formed by them and the first and second pitching axes is also changed. These changes may therefore be used to provide secondary controlling functions that give further desirable modifications of the first and/or second hydrofoil members characters and of the ways they interact.
Secondary controlling functions may include, for example, varying the strength of the opposition means by either; retaining the same regulated angle, but altering the effect of a strut that is providing opposition, (e.g. by moving at least one of its attachment locations on its respective hydrofoil member); or by retaining the same strut attachment locations, but altering the regulated angle.
Also, secondary controlling functions such as, for example, the movement of the strut attachment locations on their respective hydrofoil members, as mentioned above, and/or changing the pitching moment and/or lift and drag characteristics of at least one of the hydrofoil members, may be controlled through an interactive effect which takes place when the angle formed by at least one of the first and second pitching axes, and a bridle member, is altered, through at least one bridle member length adjustment. It may then be desirable that modulation means are employed whereby these secondary controlling functions are modulated, in varying proportionate ways, to work more appropriately with the primary controlling functions that they accompany. It is also desirable that secondary controlling functions provided in this way are unaffected by pitching of the hydrofoil member concerned, or at least, are affected only to an extent that is desirable.
To minimise the drag of bridle members and or tow-lines, at least a part of at least one of the bridle members, and/or tow-line may be of an aero hydrodynamically faired cross-section. Such a faired cross-section may be of a super-cavitating type.
Equipment such as, for example, controlling mechanisms, activating devices, power sources and any special equipment may all be housed within any of the members of the hydrofoil apparatus and/or attached to its bridle and/or towlines. Also power; control information and/or data information may be passed along at least one of its bridle members and/or tow-line, and control information and or data information may be passed by other remote means.
At least two of the constituent members of the hydrofoil apparatus may be easily disassembled, in order to facilitate its handling and stowage.